Shadow device for a gas laser

ABSTRACT

An electrode arrangement for a gas laser is provided. The electrode arrangement includes an elongated high voltage electrode, an elongated ground electrode disposed adjacent to the high voltage electrode, a discharge gap between the two electrodes an insulator element, a high voltage conductor having a first end connected to the high voltage electrode and extending through the insulator element, and a shadow plate interposed between the discharge gap and the insulator element. The electrode arrangement may be employed in a variety of gas lasers, including excimer lasers.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention relates to gas discharge lasers, and in particular to excimer lasers, and more particularly to an electrode arrangement for such gas discharge lasers.

2. Background Of The Invention

Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as for other industrial applications.

Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example ArF excimer laser) or a gas containing a halide (for example F₂) as the laser gas.

Generally, in an excimer laser. a gas mixture containing the active component and other gases is steadily provided to a discharge gap between a pair of elongated electrodes inside the laser tube by means of a fan or the like. A high voltage applied between the two electrodes causes a gas discharge in said discharge gap, whereby, from the active component of the gas, short-lived excited-state molecules are generated, whose disassociation generates ultraviolet radiation constituting the laser radiation. To increase the homogeneity of the gas discharge, in present excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be reused, excimer lasers are generally operated in a pulsed operation mode, wherein the laser gas in the discharge gap is being steadily replaced by fresh or regenerated laser gas provided by the fan.

The discharge electrodes of an excimer laser are usually located inside the laser tube.

The housing of an excimer laser generally consists of a metal tube having openings in a cylindrical wall on the upper side thereof. An insulating plate covers the open upper side. The metal tube and one of the discharge electrodes are grounded. A high voltage is applied to the second discharge electrode via a HV duct extending through the insulating plate.

One main problem of excimer lasers, which is still not satisfactorily solved, is the contamination of the laser gas due to the corrosive effect of the active components of the laser gas on many insulating materials which are widely used as insulators, especially on materials containing carbon molecular structures, such as many plastic materials, for example TEFLON®. Due to this contamination the lifetime of the laser gas is reduced, which makes a frequent exchange of the laser gas necessary. To overcome this problem, U.S. Pat. No. 4,891,818 utilizes high-purity aluminum oxide (Al₂O₃) as insulator, on which the corrosive effect of the active components of the laser gas is by far reduced as compared to plastic materials.

Another, even more corrosion-resistive, material that can be used as insulators are fluorides.

However, even though corrosion-resistive materials are used, corrosion cannot be avoided, in particular in those that are subjected to the very aggressive laser light. However, the production of laser light cannot be avoided, as this is, of course, the purpose of the laser. To date, no attempt or suggestion has been made regarding how to protect the insulators from the enhanced corrosion resulting from exposure to the laser light.

A need therefore exist for a mechanism to protect insulators from the aggressive corrosion resulting from exposure to the laser light. In addition, a need exist for a mechanism to protect the insulators from the light generated by the gas discharge and from the pre-ionizers, as such light also results in enhanced corrosion of the insulators. Such a mechanism would not only extend the lifetime of the insulators, but also, and perhaps more importantly, significantly lower the dust in the gas mixture within the laser tube.

Directly related to the above problem is the problem, that exchanging of the gas and maintenance works are expensive and time-consuming. Moreover, they are hazardous activities, as the laser gases for excimer lasers are, besides their corrosive nature, highly toxic.

RELATED APPLICATIONS

The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:

Docket Serial or No. Title Inventors Filing Date Patent No. 249/300 Gas Laser Discharge Unit Claus Strowitzki February 22, and Hans Kodeda 2000 249/301 A Gas Laser and a Hans Kodeda, February 22, Dedusting Unit Thereof Helmut Frowein, 2000 Claus Strowitzki, and Alexander Hohla 249/302 Dedusting Unit for a Laser Claus Strowitzki February 22, Optical Element of a Gas 2000 Laser and Method for Assembling 249/304 Modular Gas Laser Claus Strowitzki February 22, Discharge Unit and Hans Kodeda 2000 250/001 Adjustable Mounting Unit Hans Kodeda, February 22, for an Optical Element of Helmut Frowein, 2000 a Gas Laser Claus Strowitzki and Alexander Hohla 250/002 An Optical Element Hans Kodeda and February 22, Holding and Extraction Helmut Frowein 2000 Device

All of the foregoing applications are incorporated by reference as if fully set forth herein.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an electrode arrangement for a gas laser, and in particular for an excimer laser, that minimizes contamination of the laser gas and thus increases the lifetime of the laser gas.

Another object of the present invention is to provide an electrode arrangement for a gas laser and in particular for an excimer laser that is easy to handle and yet powerful.

The above and further objects of the invention are achieved by an electrode arrangement for a gas laser comprising an elongated high voltage electrode, an elongated ground electrode disposed adjacent to the high voltage electrode, a gas discharge gap between the two electrodes in which the gas discharge for the laser is generated, an insulator element, a high voltage conductor extending through the insulator element and having a first end connected to the high voltage electrode, and a shadow plate interposed between the gas discharge gap and the insulator element for shielding the insulator element against laser radiation irradiated from the gas discharge gap, as well as from gas discharge radiation and pre-ionization radiation.

The shadow plate is preferably made of a high purity metal, such as aluminum. Alternatively, it can be made of an insulator material, such as a heat resistant plastic material, such as TEFLON®, or a ceramic material, which is then covered by a metal, preferably of high purity. According to still a further alternative the shadow plate can be made of any other material that is resistant against the laser gas and the laser radiation, gas discharge radiation, and pre-ionizer radiation.

The shadow plate can be arranged at any location between the gas discharge gap and the insulator element. For example, it can be located directly above the discharge gap on the high voltage electrode, or it can be located on the high voltage conductor. Preferably, the shadow plate is interposed between the high voltage electrode and the high voltage conductor.

The insulator may comprise an elongated shape and extend continuously substantially over the entire length of the laser tube and thus form a prior art electrode plate, which serves as an upper cover for the laser tube, and through which the high voltage electrode extends.

Preferably, however, the insulator element comprises a more compact form. Thus, an electrode arrangement according to a preferred embodiment according to the present invention comprises an elongated electrode plate made of an electrically conductive material and having a plurality of spaced-apart holes therein and a plurality of waveguide-like coaxial high voltage ducts, wherein each duct extends through one of the holes in the electrode plate. Each high voltage duct comprises a central conductive core having a first end and an insulator element, preferably made of a ceramic material. The insulator element is disposed around the core, preferably in a concentrical manner, to electrically insulate the core and electrode plate from each other. The electrode arrangement according to the preferred embodiment further comprises an elongated high voltage electrode electrically connected to the first end of the cores of the ducts, an elongated ground electrode positioned to oppose the high voltage electrode and form a gas discharge gap therebetween, the ground electrode being electrically connected to the electrode plate, and a shadow plate interposed between the gas discharge gap and the insulator element of each of the ducts.

As the electrode plate is grounded, an insulator is required between the conductive cores of the HV ducts and the electrode plate. The number of high voltage ducts (and holes) depends on the size of the laser, in particular on the length of the electrodes. For example, for a typical excimer laser, three high voltage ducts should be used. For a larger laser with longer electrodes more than three ducts should be provided. For smaller lasers with shorter electrodes only one or two ducts may be provided.

For practical reasons, the coaxial ducts comprise a round cross-section so that commercially available parts can be used. In principle, however, the ducts can comprise any cross-sectional shape, such as square or rectangular or oblong or anything else.

The shadow plates can be flat. Preferably, however, the edges of the shadow plate are bent towards the insulator elements. The edges of the shadow plate are preferably bent towards the insulator element for ensuring more efficient shading of the insulator. Furthermore, the shadow plate desirably comprises a flow-guiding shape to help guide the laser gas mixture, which typically comprises the laser gas and a buffer gas into the gas discharge gap.

The preferred discharge unit further comprises a pair of standard corona pre-ionizers, that is a pair of elongated cylindrical pre-ionizers with a conductive core and a surrounding tube-shaped insulator. The pre-ionizers extend substantially parallel along opposite sides of the electrode. The insulator of the pre-ionizers can be TEFLON®, or any suitable insulator, but it is preferably a ceramic material, such as aluminum oxide. The insulator can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable.

The overall construction of the laser can be such that first a laser tube is provided, and then the high voltage electrode, the ground electrode, the insulator element or elements, the high voltage conductor or conductors, the pre-ionizers and the shadow plate are mounted to the tube, one by one. It is preferred, however, that the electrode arrangement is a pre-mounted module-type discharge unit, wherein the electrodes, the shadow plate, the pre-ionizers and the high voltage ducts are pre-mounted independently of other laser elements. In this manner, the discharge unit can be mounted to the laser tube as a whole. This provides several advantages. One advantage is that the gas discharge gap between the high voltage electrode and the ground electrode can be adjusted before the discharge unit is mounted into the laser tube. This facilitates an accurate adjustment of the gas discharge gap. Furthermore the mounting of the laser can be done in a more efficient manner.

The laser gas can, in the case of an excimer laser, be any excimer laser gas, such as KrF, ArF, XeF, XeBr, HgBr, HgCl, XeCl, HCl, F₂, Ar₂ and the like or any laser gas in case of some other gas discharge laser.

Besides the laser gas, a buffer gas comprising a mixture of Helium, Neon and/or Argon is preferably provided in the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cut-away side view of an excimer laser according to a preferred embodiment of the invention;

FIG. 2 is a cross-sectional view of the excimer laser in FIG. 1 along Line 2—2;

FIG. 3a shows a side view of a discharge unit according to a preferred embodiment of the invention;

FIG. 3b shows a front view of the discharge unit of FIG. 3a;

FIG. 3c shows a plan view of the discharge unit of FIG. 3a;

FIG. 4 shows a detailed cross section of the discharge unit according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following a preferred embodiment of the invention will be set forth.

FIGS. 1 and 2 show an excimer laser 100 comprising a tube 101, a discharge unit 102, a circulation means 201, and a laser optical system 103. As is known in the art, gas laser 100 may also comprise means for cooling the laser gas, such as a gas cooler, within laser tube 101.

The circulation means 201 is optional and may comprise, for example, a fan or any other means known for circulating lasing gases in gas lasers.

The discharge unit 102 is mounted into the tube 101 and comprises a high voltage electrode 104 and the ground electrode 105. The high voltage electrode 104 and the ground electrode 105 are spaced apart from each other, thereby defining a gas discharge gap 106. A high voltage is applied to the high voltage electrode 104 via a plurality of high voltage ducts 107, which carry the high voltage electrode 104. Each high voltage duct 107 comprises a conductive core 108 and an insulator element 110 arranged around the conductive core 108. Each high voltage duct 107 is attached to the high voltage electrode 104. The high voltage ducts may be attached to the high voltage electrode using any suitable fastener. In the present embodiment, a double threaded stud 112 is used to attach electrode 104 to each conductive core 108 of each duct 107.

Furthermore, discharge unit 102 is provided with an elongated electrode plate 111. The electrode plate 111 includes holes, through which the high voltage ducts 107 extend so as to be connected to the high voltage electrode 104. Each high voltage duct 107 is fixed to the electrode plate 111 by an attachment means, such as bolts 113. Those skilled in the art will appreciate, however, that any suitable attachment means may be used to fix ducts 107 to electrode plate 111.

The insulator elements 110 are preferably made of a ceramic material. Optionally, however, they may be made from other insulative materials, including, for example, a fluoride material. They preferably have a shape that conically expands towards the high voltage electrode 104 and comprise a corrugated surface, so as to increase a creepage path extending along said surface. This helps to prevent surface flashover between the high voltage electrode 104 and the grounded electrode plate 111.

As noted above, insulator elements 110 may be made from fluoride insulator material. These materials have the drawback of being relatively expensive. However, according to the preferred embodiment of the present invention, only a small amount of insulator material is required. Accordingly, the use of fluoride insulator materials is affordable in the present invention.

As illustrated in FIG. 2, discharge unit 102 also preferably comprises a shadow plate 210 disposed between the gas discharge gap 106 and the insulator element 110 for protecting the insulator element 110 against the corrosive effects of the laser gas and of the laser, gas discharge, and pre-ionizer radiation. Shadow plate 210 is preferably made out metal, such as aluminum. A high purity metal is particularly preferred for use in constructing shadow plate 210.

In the present embodiment, shadow plate 210 is interposed between the high voltage electrode 104 and insulator element 110 of the ducts 107. Preferably shadow plate 210 extends along the entire gas discharge gap 106 and is mounted in such a manner that it at least partially shields the insulator element against the laser radiation irradiated from the gas discharge gap 106.

Referring to FIGS. 2 and 3a, shadow plate 210 preferably has an elongated sheet-like shape and comprises a central portion 209, a first edge portion 211 and a second edge portion 212. The central portion 209 extends longitudinally in a direction parallel to the gas discharge gap 106 and perpendicular to the cores 108 of the high voltage ducts 107. The edge portions 211 and 212 are located at the longitudinal edges of the shadow plate 210 and are, with respect to the central portion 209, preferably bent towards the insulator elements 110 by a small angle of about 20 degrees. Although shadow plate 210 is preferably elongated as described above, it may have a variety of other shapes as well. For example, a plurality of shadow plates 210 could be used instead of one elongated shadow plate that extends the length of the discharge gap. In such a case, the shadow plates would preferably be circular and have a cross-sectional appearance consistent with that shown in FIG. 2. Thus, a circular shadow plate 210 could be interposed between the electrode 104 and each insulator element 110 of the high voltage ducts 107.

The type of laser gas employed may also affect the shape of the shadow plate 210. The reason for this is that some laser gases have a substantially lower breakdown voltage than other laser gases. For example, the laser gases used in ArF and KrF excimer lasers have a substantially lower breakdown voltage than the F₂ gas used in fluorine gas lasers. Thus, if laser 100 is a fluorine gas laser, then the shadow plate 210 may come much closer to the insulator element 110 than if laser 100 is an ArF or KrF excimer laser. As a result, bending the edges of the shadow plate 210 toward insulator element 110 may be appropriate. On the other hand, bending the edges of shadow plate 210 toward the insulator element for other excimer lasers may be inappropriate. Indeed, for example, with ArF, KrF, and other excimer lasers employing laser gases with relatively low breakdown voltages, it may be advantageous to bend the edges of the shadow plate away from the insulator element 110 to help ensure that current does not jump the gap between the shadow plate and insulator element. However, this of course, will depend on how far the shadow plate 210 is spaced from the insulator element 110 and the breakdown voltage of the laser gas being used. Similarly, the greater the breakdown voltage of the laser gas, the smaller the diameter of the conical portion of the insulator element 110 may be.

As best seen in FIG. 2, the shadow plate 209 may be interposed between the high voltage electrode 104 and inner ends 404 of the cores 108 of the high voltage ducts 107 so that the longitudinal axis of the central portion 209 (or center in the case of a circular shadow plate) coincides with the central axis of the high voltage electrode 104.

Although shadow plate 210 is preferably interposed between high voltage electrode 104 and inner ends 404 of the cores 108 as illustrated in FIG. 2, as those skilled in the art will appreciate, the shadow plate 210 will serve its desired function so long as it is interposed between the discharge gap 106 and insulator elements 110. Thus, the positioning of shadow plate 210 is not limited to the position illustrated in FIG. 2.

Shadow plate 210 may be interposed between electrode 104 and cores 108 by providing central portion 209 with a plurality of holes 213, preferably along the longitudinal axis of the central portion 209, and then attaching high voltage electrode 104 to the cores 108 through holes 213 using a suitable fastener. Thus, the spacing and the number of holes 213 correspond to the spacing and the number of high voltage ducts 107. In the present embodiment, stud bolts 112 with threads on both ends are used to attach electrode 104 to cores 108. One end of each stud bolt 112 is inserted into a threaded hole 124 provided in the respective core 108. The second end of the stud bolt 112 is inserted into a further threaded hole 126 provided in the mating face 128 of the high voltage electrode 104 that faces the inner end 404 of core 108 and the shadow plate 210. If a circular shadow plate is used, each shadow plate will be provide with a single hole 213 in the center of the shadow plate and one shadow plate will be used for each high voltage duct employed in the laser.

The shadow plate 210 preferably has a flow-guiding shape to help guide the lasing gas mixture into the gas discharge gap 106.

A preferred manner of assembling the electrode arrangement of the present invention with the shadow plate is now described.

First one end of a stud bolt 112 is screwed into each of the threaded holes provided on the mating face 128 of the high voltage electrode 104, such that the other end of each stud bolt 112 stands out of the mating face 128. Then the shadow plate 210 is arranged on the mating face 128 of the high voltage electrode 104 so that the stud bolts 112 are inserted into the holes 213 in the'shadow plate 210. Alternatively, if a circular shadow plate is used, then one shadow plate 210 will be inserted over each of the stud bolts 112. After the shadow plate 210 is in place, a core 108 of the high voltage ducts 107 is lowered upon the shadow plate 210 such that the end of one of the stud bolts protruding from the electrode partially enters the threaded hole 124 provided in the inner end 404 of the core 108 of the high voltage duct 107. Subsequently the core 108 is rotated around its longitudinal axis, i.e. around the longitudinal axis of the stud bolt 112, so as to screw the core 108 onto the stud bolt 112. As a result, the core 108 is lowered onto the shadow plate 210, and the shadow plate 210 is finally held between the upper face 128 of the high voltage electrode 11 04 and the inner end 404 of the core 108. Additional high voltage ducts 107 comprising cores 108 are attached to the remaining stud bolts 112 in the same way as described above.

In the case of an elongated shadow plate, before the cores 108 are tightly screwed to the stud bolts 112, at least two of the cores 108 are loosely screwed to their corresponding stud bolt 112. Then, after the shadow plate 210 is correctly positioned, all of the cores 108 are screwed down tightly to lock shadow plate 210 in place.

The excimer laser 100 may be, for example, a pulsed fluorine gas (F₂) excimer laser with a wavelength of about 157 nanometers. This means that fluorine gas is used for generating the laser beam. However, as those skilled in the art will appreciate, any of the known excimer laser gases may be used in connection with the present invention.

By applying a high voltage pulse on the order of 20 kV to the high voltage electrode 104, the laser gas (e.g., fluorine gas) and additionally helium and/or argon gas as a buffer gas in the discharge gap 106 generate a laser beam which is emitted through the laser optical system comprising a front optical system 103 and a rear optical system 120.

Laser 100 typically further comprises a front optical element 116, through which the laser beam emits. Optical element 116 may be provided, for example, in an optical system 103 that includes an adjustable mounting means 117 for adjusting the position of the optical element 116 in relation to the tube 101. Rear laser optical system 120 similarly includes an optical element 116 (not shown) and adjusting means 117. However, the optical element 116 of the rear laser optical system 120 comprises a totally reflective mirror rather than a partially reflective mirror. As those skilled in the art will appreciate, front and rear optical elements 116 may also be mounted directly in the end walls of the laser tube 101. Alternatively, they may be mounted on adjustable mounting brackets that are separate from the laser tube 101 as is known in the art. A suitable laser optics system and an adjustable mounting means for use in connection with the present invention as front and rear optical systems 103, 120 are described in concurrently filed applications bearing attorney docket nos. 250/001 and 250/002, which are hereby incorporated by reference. The filing details of these applications are provided above.

FIG. 2 is a cross-sectional view along line 2—2 of the excimer laser 100 shown in FIG. 1. As can be seen in FIG. 2, the excimer laser 100 preferably further includes a circulating means 112, such as a fan, for circulating the excimer laser gas through the discharge gap 106 and an optional dedusting unit 202 for dedusting the gas flow through the tube 101. The dedusting unit comprises high voltage wires 203, separated from each other by U-shaped channels 204 extending along the tube 101. Furthermore, two guiding plates 205, which are elongated in the longitudinal direction of the tube 101 are preferably provided for guiding the gas flow through discharge gap 106 and a portion of such gas into dedusting unit 202. After exiting dedusting unit 202, the gas returns to fan 201 to be recirculated through the laser 101. A detailed description of a suitable dedusting unit 202 for use in connection with the present invention is provided in a concurrently filed application bearing attorney docket no. 249/301, which is hereby incorporated by reference. The filing details of this application are provided above.

The ground electrode 105 is preferably carried by, or mounted to, the electrode plate 111 via a plurality of flow guides 209, which will be referred to again later.

Adjacent to the high voltage electrode 104, two pre-ionizers 206 are provided, which serve to pre-ionize the laser gas to ensure greater homogeneity of the gas discharge in the discharge gap 106.

The pre-ionizers 206 are preferably corona-type pre-ionizers and extend substantially parallel to said high voltage electrode. The pre-ionizers 206 have a coaxial shape with a conductive core 207 surrounded by a tube shaped insulator 208.

The corona-type pre-ionizers can be mounted immediately adjacent to the high voltage electrode. In particular, as shown in FIG. 2, the corona-type pre-ionizers should be mounted at the opposing edges of the high voltage electrode so that it is disposed adjacent the electrode face of the high voltage electrode facing the ground electrode.

Although corona-type pre-ionizers are preferred for use as pre-ionizers 206 in connection with the present invention, those skilled in the art will recognize that any of the pre-ionizers known in the art may be used. Furthermore, the insulator of the pre-ionizers is preferably a ceramic material such as alumina. It can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable.

Referring to FIGS. 3a and 3 c the discharge unit 102 comprises three coaxial waveguide-like high voltage ducts 107, extending through holes in the electrode plate 111. The ducts 107 are arranged spaced apart from each other. The holes and the ducts 107 have a circular cross section, as can be seen from FIG. 3c. Each of the three ducts 107 is inserted into the respective hole in the electrode plate 111 with a defined tolerance between the insulator element and the hole. As those skilled in the art will appreciate, the number of ducts employed in a particular gas laser 100 will depend on the overall length of the laser.

The ground electrode 105 is preferably carried by, or mounted to, the electrode plate 111. As best seen in FIGS. 2 and 3a, preferably a plurality of flow guides 209 are used for this purpose.

The flow guides 209 are preferably made from sheets of metal that extend between the electrode plate and the ground electrode in a plane perpendicular to the longitudinal axis of the electrodes 104, 105. The flow guides plates 209 comprise respectively an upper flange 301, a lower flange 303, and a central flow-guiding portion 302 integrally connecting said upper flange 301 to said lower flange 303. Said upper and lower flanges 301, 303 extend perpendicular to each other and to said central flow-guiding portion 302. The upper flange 301 is attached to a side face 304 of electrode plate 111, and the lower flange 303 is attached to a bottom face 305 of the ground electrode 105. The central flow-guiding portion 302 is preferably aerodynamically profiled in order to minimize flow resistance and turbulences for maintaining a substantially laminar gas flow between the flow guides.

The lower flange 303 preferably includes an oblong hole 306 (shown only at part of the flow-guiding plates 209). Hole-306 is oblong in a direction perpendicular to the longitudinal axis of the elongated ground electrode 105. A screw, or other fastening means, 307 is inserted through the hole 306 into a mating threaded hole 308 being provided in the ground electrode 105. The oblong hole 306 allows for adjustments of the ground electrode 105 with respect to the high voltage electrode 104 essentially in the direction indicated by the double-headed arrow 320 in FIG. 3c.

The upper flange 301 preferably includes an oblong hole 309. Hole 309 being oblong in a direction perpendicular to the longitudinal axis of the electrode plate 111. A screw, or other fastening means, 310 is inserted through the hole 309 into a mating threaded hole 311 being provided in the high voltage electrode 104. The oblong hole 309 allows an adjustment of the ground electrode 105 with respect to the high voltage electrode 104 essentially in the direction indicated by the double-headed arrow 322 in FIG. 3a.

FIG. 4 shows a cross section of the discharge unit 102 according to the preferred embodiment of the invention. In particular, FIG. 4 shows an enlarged cross-sectional view of the discharge unit shown in FIG. 2. The angle of view is the same as in FIG. 3b.

Each high voltage duct 107 of the laser discharge unit 102 preferably further comprises a sleeve 401 enclosing the core 108 and insulator 110. Sleeve 401 has an inner end 402 supported by the electrode plate 111, and an outer free end 403. The core 108 has an inner end 404 connected to the high voltage electrode 104 and a threaded outer free end 405 extending beyond the free end 403 of the sleeve 401. A nut 406 may be screwed onto the threaded end 405 as shown in FIGS. 3c and 4, thereby pressing the sleeve 401 against the electrode plate 111 and tensioning the core 108 by pulling it. Preferably a washer 450 is interposed between nut 406 and insulator 110 to evenly distribute the stresses applied by nut 406 to insulator 110. A threaded stud bolt 112 is used to connect the inner end 404 of the core 108 to the high voltage electrode 104.

The inner end 404 of the core 108 is provided with a core ring shoulder 408, which is pressed against the ceramic insulator element 110 when core 108 is placed under tension. A seal 409 is preferably provided between the ring shoulder 408 and the ceramic insulator element 110.

The ceramic insulator element 110 is also caused to be pressed against the electrode plate 111 by means of the tensioned core 108 via the core ring shoulder 408 at the inner end 404 of the core 108. Preferably a ring shoulder 410 is provided on the insulator element 110 and another seal 411 is provided between the ceramics insulator ring shoulder 410 and the electrode plate 111.

To provide additional sealing, a sealing ring 412 (see also FIGS. 2 and 3c) preferably surrounds each sleeve 401. Sealing ring 412 may be constructed to have a flange 413 at its outer circumference. Flange 413 is dimensioned so that it is supported by an outer rim 414 of the holes 150 in the tube 101 through which the respective ducts 107 are inserted. Electrode plate 111 is then preferably provided with a ring shoulder 417 facing an inner rim 415 of the tube 101. A metal seal 416 is preferably interposed between shoulder 417 and rim 415. As a result, when the ring 412 and the electrode plate 111 are connected by screws 113 a gas tight seal is provided between the shoulder 417 and the inner rim 415 of the tube 101.

All of the seals 409, 411 and 416 are ring-shaped metal seals in the present embodiment. However, those skilled in the art will appreciate that the invention is not limited to using ring-shaped seals.

As will be understood by one skilled in the art, the invention may be embodied in other specific forms without departing from the spirit and the scope of the invention. The description of the embodiment is given as an illustrative example only and should not be understood as a limitation of the invention, which is set forth in the following claims. 

We claim:
 1. An electrode arrangement for a gas laser, comprising: an elongated electrode plate made of an electrically conductive material and having a plurality of spaced-apart holes therein; a plurality of coaxial high voltage ducts, each duct extending through one of the holes in said electrode plate, and each duct comprising a central conductive core having a first end and an insulator element disposed around said core and electrically insulating said core from said electrode plate; an elongated high voltage electrode electrically connected to the first end of said cores of said ducts; an elongated ground electrode positioned to oppose said high voltage electrode and form a gas discharge gap therebetween, said ground electrode being electrically connected to said electrode plate; and a shadow plate interposed between the gas discharge gap and the insulator element of each of said ducts.
 2. An electrode arrangement according to claim 1, wherein said shadow plate is interposed between said high voltage electrode and the first end of said core of each of said ducts.
 3. An electrode arrangement according to claim 1, wherein said shadow plate includes an edge bent towards said insulator element.
 4. An electrode arrangement according to claim 1, wherein said shadow plate comprises a flow-guiding shape shaped to guide gas flowing across its surface into said discharge gap.
 5. An electrode arrangement according to claim 1, wherein said electrode plate, said high voltage ducts, said electrodes, and said shadow plate are assembled together to form a removably mountable module.
 6. An electrode arrangement according to claim 5, wherein said shadow plate is interposed between said high voltage electrode and said high voltage ducts.
 7. An electrode arrangement according to claim 5, wherein said shadow plate comprises an edge bent towards said insulator element.
 8. An electrode arrangement according to claim 5, wherein said shadow plate comprises a flow-guiding shape that is shaped to guide gas flowing across its surface into said discharge gap.
 9. An electrode arrangement according to claim 1 which is adapted to be used in an excimer laser.
 10. An electrode arrangement according to claim 1, wherein said shadow plate has an elongated shape and extends substantially the entire length of said discharge gap.
 11. An electrode arrangement according to claim 1, wherein said shadow plate has a circular shape.
 12. An electrode arrangement according to claim 1, wherein said shadow plate is metal.
 13. An electrode arrangement according to claim 1, wherein said insulator has a concentric shape.
 14. An electrode arrangement according to claim 5, wherein said insulator has a concentric shape.
 15. A gas laser, comprising: a sealed tube containing a gas mixture comprising a laser gas; a discharge unit removably mounted inside said tube, said discharge unit comprising an elongated electrode plate made of a conductive material and having a plurality of spaced-apart holes therein, a plurality of coaxial high voltage ducts, each duct extending through one of the holes in said electrode plate and comprising a central conductive core and an insulator element being arranged around said core and electrically insulating said core from said electrode plate, an elongated high voltage electrode connected to said cores of said ducts, an elongated ground electrode adjustably mounted to said electrode plate so that it is spaced-apart from and substantially parallel to said high voltage electrode, and a shadow plate interposed between the discharge gap and the insulator element of each of said ducts; circulation means disposed within said tube; a first laser optical element disposed at one end of said discharge gap; and a second laser optical element disposed at the opposing end of said discharge gap.
 16. A gas laser according to claim 15, wherein said shadow plate is interposed between said high voltage electrode and said conductive core.
 17. The gas laser according to claim 15, wherein said insulator element has an elongated shape and extends along substantially the entire length of said electrodes.
 18. The gas laser according to claim 15, wherein the shadow plate includes an edge bent towards said insulator element.
 19. The gas laser according to claim 15, wherein said shadow plate comprises a flow-guiding shape that is shaped to guide gas flowing across its surface into said discharge gap. 